1. Field of the Invention
This invention relates to a hot melt type ink jet apparatus, and more particularly to fixation of ink on printing paper.
2. Description of Related Art
Ink jet printers, including the so-called hot melt type, are designed to eject liquid ink against printing paper to record characters and patterns on the paper. Hot melt type ink jet printers are arranged to heat and melt a normally solid ink at the time of printing and to eject the molten ink to print a character. Generally, hot melt type ink jet printers include (a) a platen which serves to support printing paper, (b) an ink supplying device which is adapted to store a normally solid ink therein and is provided with an ink heater for melting the ink, and (c) an ink ejection head which ejects molten ink from the ink supplying device to print a character on printing paper. Printers of this sort can make clear prints free of ink blurring.
However, since the ink droplets or globules which are ejected from the ink ejection head are extremely small in size and thus in heat capacity, they immediately lose their heat and solidify as soon as they are deposited on printing paper, forming a ridged lump of ink on the surface of the printing paper. Such an ink lump is apt to be scraped off or peeled from the printing paper when rubbed against other printing paper or scratched by an operator's hand. This makes printed characters unclear, and additionally creates a problem of smudging the back sides of overlying printed paper sheets when a number of printed paper sheets are piled one on another.
A technology for solving these problems is disclosed in Japanese Patent Application laid open No. 62-161542, which concerns a hot melt type ink jet printer which is provided with a pressing roller for applying pressure to a printed paper sheet. A printed portion of printing paper is gripped and pressed between the pressing roller and a paper-supporting platen both of which are formed of rigid material such as stainless steel, thereby flattening the ridged ink lump to some extent. A similar technology is also disclosed in U.S. Pat. No. 4,745,420.
The stainless steel pressing roller as employed in the ink jet printer of the above-mentioned Japanese patent publication, however, has a problem that the ink easily adheres to the pressure roller surface due to high cohesiveness of the metallic surface. It follows that the ink lump readily adheres to the pressing roller when pressed by the latter, as a result thinning the density of the printed character and contaminating the pressure roller surface which results in smudging of printing paper by transfer of the ink from the contaminated surface when the roller is later brought into contact with a newly printed paper surface.
Additionally, as long as the pressing roller is formed of a rigid material, including metal or non-metallic materials, there is a problem that the ink lumps are cracked in peripheral portions as they are crushed by the pressure roller and caused to rise up from the printing paper increasing the possibility of peeling due to lowered adhesion.